1. Field of the Invention
This invention relates to methods of cleaning cryofilms and cryofilm/organic mixtures from cryogenically cooled surfaces, and more particularly to the use of plasma cleaning for removing such contaminants without damaging an underlying optical quality surface.
2. Description of the Related Art
Cryo-telescopes, which are typically used for spacecraft applications, have optical surfaces that are cooled to temperatures in the range of tens Kelvin to make observations in the IR regime. Such devices suffer from a build up of surface contaminants that absorb incident radiation and over time become roughened by sublimation roughening, thereby increasing their optical scatter. The contaminants consist primarily of a mixture of frozen gases such as nitrogen, oxygen, water, ammonia and carbon dioxide that are normally trapped in the materials of the spacecraft, and organic compounds that are outgassed from organic elements in the spacecraft such as adhesives, potting compounds, conformal coatings, bonding agents and thermal blankets. The frozen gases are generally referred to as cryofilms. At temperatures less than approximately 150 K the primary component of contamination is usually a water cryofilm.
In the past, cryo-telescopes have been cleaned by warming them to sublime the cryofilms. However, this process renders the telescope unusable during the time it is being performed, consumes a great deal of cryogen, and allows the residual hydrocarbon contaminants to continue to accumulate.
Another approach to cleaning spacecraft surfaces is described in U.S. Pat. No. 4,846,425 to Champetier and assigned to Hughes Aircraft Company, the assignee of the present invention. This technique uses the negative charge that is typically accumulated on a spacecraft and collects more active electrons than relatively inactive positive ions. Neutral oxygen is released from the spacecraft, ionized by the background space plasma, and drawn back to the spacecraft by its negative charging to react with the surface contaminants. A very large oxygen supply is required, however, because most of the oxygen escapes and is not drawn back to the spacecraft. The patent discloses the reactive ion cleaning of organic contaminants, but not cryofilms. The low energy levels which it uses (about 1-10 eV) would normally be considered too low to produce a sputtering effect capable of removing a cryofilm.
Other attempts to clean space-borne optical devices have employed high energy ion beams. While this technique does result in the removal of contaminants, it has caused unacceptable damage to the delicate optical quality surfaces by ion beam sputtering. The approach is discussed in Lippey and Gleichauf, xe2x80x9cOn-Orbit Ion Cleaning of Cryogenic Optical Surfacesxe2x80x9d, SPIE, Vol. 1754, 1992, pages 314-323. 
Electron cleaning has also been used to remove cryofilm contamination, as described in Pyier et al, xe2x80x9cCROSS: contaminant removal off optical surfaces in spacexe2x80x9d, SPIE, Vol. 777, 1987, pages 320-332. Unfortunately, electron cleaning has not been found to consistently remove organic materials that may be present along with the cryofilm contamination unless the background level of oxygen gas is high enough (approximately 1xc3x9710xe2x88x925 Torr) to result in oxidation rather than polymerization or carbonization. Electron cleaning also heats the surface being cleaned, thus forcing the cleaning rate to be limited by the spacecraft""s cryogenic cooling capacity.
The present invention seeks to provide a method for cleaning cryofilms and cryofilm/organic mixtures from delicate cryogenically cooled surfaces that avoids damage to the underlying surface, allows the device to continue operating during the cleaning process, requires only a moderate supply of the cleaning substance and avoids overheating the delicate surface being cleaned.
The inventors have discovered that, although plasmas with energies low enough to avoid damaging a delicate optical quality surface will not sputter room temperature contaminants, the same low ion energies can be used to sputter off cryofilms in the cryogenic temperature regime. The invention uses plasmas with average ion energies of not more than about 30 eV for this purpose, with the preferred average ion energy in the approximate range of 5-20 eV.
When the plasma is formed from a reactive material such as oxygen that chemically reacts with organic materials imbedded in the cryofilm, complete cleaning of the optical surface can be achieved with a single plasma. Alternately, the cryofilm and a portion of the embedded organic material can be sputtered away with a non-reactive plasma, and any residual organic material remaining on the optical surface removed by chemical reaction with a low energy reactive plasma. The latter approach is particularly useful when the substrate contaminated with the cryofilm is at a lower temperature than the freezing point of the reactive material; a low energy plasma from a non-reactive material such as helium is first used for the sputtering process, followed by a clean up with a reactive plasma such as oxygen. Any residual reactive plasma that is frozen to the substrate can then be removed by sputtering it with a second low energy sputtering plasma, which is preferably the same type of plasma used for the initial sputtering. These treatments may alternate in rapid succession to prevent buildup of a cryofilm of the reactive species. Alternatively, the inert species can contain a small admixture of the reactive species for continuous operation.
These and other features and advantages of the invention will be apparent to those skilled in the art from the following detailed description, taken together with the accompanying drawings.